New Computational Tools to Address a Hard Non-equilibrium Problem: Crystallization of Polyethylene

New simulation methods are described that can contribute to solving an old problem: understanding how molecular weight and branching distribution, rheology and processing conditions, and the resulting semi-crystallinity, control the end properties, such as modulus and toughness, of polyethylene film. In this talk, a practical method is presented that accounts for the effects of short- and long-chain branching on rheological properties of commercial polyethylene through use of an optimal ensemble of chains, that can be used to predict chain orientation in a processing flow such as film blowing. Molecular dynamics simulations are used to determine rates of primary and secondary nucleation of polyethylene crystals including the influence of branching. We show that both primary and secondary nucleation proceed through a nematic-like intermediate. We show that short-chain branches are partially expelled from the growing crystal to an extent dependent on branch length and by measuring the degree of expulsion, we can infer the defect energy induced by trapping of branches of various lengths in the crystal. Future work will apply these insights to the development of a film blowing model that includes new insights into both rheology and crystallization.